You are quite wrong. The Israelis had started ethnic cleansing of the Christians and Muslims in 1947 well before the departure of the British. The Arab troops that intervened in 1948 were attempting to prevent the Jews from continuing and finalizing the ethnic cleansing/genocide of the Christians and Muslims. There is so much propaganda on the Zionist side promulgated to Americans, it is no wonder that Americans have the point of view they have.
While not source documentation this British Forces in Palestine website, which is a neutral site, relates what actually happened:
"By the end of 1947........Jews were pursuing their 'facts on the ground' policy by attacking and destroying Arab villages and ejecting the inhabitants."
"The British are criticized for not actively defending the Arab villages but there were insufficient troops and the US had refused assistance in the form of troops or other support
(many US and other factions, including British, were actively supporting the Zionist takeover and had powerful backing). General Stockwell had specific orders that troops should only be used for defence. He was in liaison with both Arabs and Zionists and knew how strong the Haganah were (contrary to myth) and that the Palestinian Arabs, though they had plenty of courage, were badly organized, equipped and trained so stood no chance against the Zionists. Consequently, the Haganah was quickly able to occupy territory way beyond that designated by the UN."
Who was there - British Forces in Palestine